Integral silencer for electric motors

ABSTRACT

An electrical motor with integral silencer is provided having a motor housing defining an inlet aperture at one end and an expansion chamber at the other end. The expansion chamber has a reduced cross-sectional area at the entrance and exit of the expansion chamber compared to the cross-sectional area of the chamber. The exit of the expansion chamber provides an outlet aperture in the housing. A stator is situated in the housing and a rotor is rotatably mounted within the stator. A fan is secured to the rotor and rotates therewith for pulling air in the outlet aperture through the stator and exhausting the air through the expansion chamber to the housing exterior. The expansion chamber provides an impedance mismatch for the acoustic energy generated by the fan preventing a portion of the acoustic energy from exiting the expansion chamber.

This application is a continuation of application Ser. No. 07/445,504filed Dec. 4, 1989 abandoned.

BACKGROUND OF THE INVENTION

The present invention is related to the use of expansion chambers orexpansion chambers and resonators to reduce electric motor fan noise.

Large electric motors are commonly cooled with in-line fans which drawor blow cooling air across the rotor and through the stator windings. Athigh speed, these fans can produce significant levels of noise. Thenoise is typically broad-band in character due to flow turbulence withembedded tones from the fan blade passage.

In transit car applications, the traction motors can develop undesirablelevels of noise which are particularly noticeable since wheel and railnoises have been significantly reduce in modern cars due to the use ofwelded rails and rubber damped wheels.

Reactive silencers using tuned ports, resonators, and expansion chambersare the basis for designing mufflers on internal combustion engines.

It is an object of the present invention to provide electric motors withreduced noise levels at all operating speeds.

SUMMARY OF THE INVENTION

In one aspect of the present invention an electrical motor with integralsilencer is provided having a motor housing defining an inlet apertureat one end and an expansion chamber at the other end. The expansionchamber has a reduced cross-sectional area at the entrance and exit ofthe expansion chamber compared to the cross-sectional area of thechamber. The exit of the expansion chamber provides an outlet aperturein the housing. A stator is situated in the housing and a rotor isrotatably mounted within the stator. A fan is secured to the rotor androtates therewith for pulling air in the outlet aperture through thestator and exhausting the air through the expansion chamber to thehousing exterior. The expansion chamber provides an impedance mismatchfor the acoustic energy generated by the fan preventing a portion of theacoustic energy from exiting the expansion chamber.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a partial sectional side view of a prior art AC traction motorshowing the air inlet and outlet passageways and an in-line fan;

FIG. 2 is a partial sectional side view of a motor having an integralsilencer in accordance with the present invention;

FIG. 3 is a partially cutaway end view of the motor of FIG. 2;

FIGS. 4A, B, and C show a schematic representation of an electric motorin-line fan in a plenum which exhausts air through an unlined expansionchamber in accordance with one embodiment of the present invention, thecorresponding impedance-type analogous circuit for the schematicrepresentation, and a graph of the transmission loss as a function offrequency for the schematic representation, respectively;

FIGS. 5A, B and C show a schematic representation of an electric motorin-line fan in a plenum which is exhausted through an expansion chamberlined with porous acoustic material in accordance with anotherembodiment of the present invention, an impedance-type analogous circuitrepresentation of the schematic representation, and a graph showing thetransmission loss versus frequency for different damping values producedby the lining in the schematic representation, respectively; and

FIGS. 6A, B and C show a schematic representation of an electric motorin-line fan in a plenum which exhausts air through an unlined expansionchamber with a lined resonator situated out of the main air stream inaccordance with the present invention, an impedance-type analogouscircuit representation of the schematic, and a graph showingtransmission loss versus frequency with and without the lined resonatorpresent respectively.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawing wherein like numerals indicate lineelements throughout and particularly FIG. 1 thereof, the prior art ACtraction motor is shown having an in-line cooling fan 11. Duringoperation, the fan rotates with the rotor 13, directing the air pulledin through cooling ducts 15 at one end of the housing 17 through thestator winding 21 and the gap between the rotor 13 and the stator 21,out the exhaust ducts 23 disposed radially outwardly from the fan 11.Cooling air is also drawn through the ducts 25 at the opposite end ofthe motor and directed radially outwardly by the fan. The fan at highspeeds produces significant levels of noise.

Referring now to FIGS. 2 and 3 an AC motor with an integral silencer inaccordance with the present invention is shown. Air is drawn into themotor through inlet ducts 15 and passes through the stator 21 and in theair gap between the stator and the rotor 13 and is directed by the fan11 through an expansion chamber 27 to the outside atmosphere. Theexpansion chamber 27 communicates with a lined resonating chamber 31which is located outside the air flow path. The expansion chamber 27 hasa plurality of inlet and outlet passageways 33 and 35, respectively. Thetotal crosssectional area of the inlet passageways is less than thecross-sectional area of the expansion chamber. The total cross-sectionalarea of the outlet passageways is also less than the cross-sectionalarea of the expansion chamber. The outlet passageways 35 of theexpansion chamber 27 also serve to exhaust the cooling air from thehousing. The resonating chamber is lined with a porous acoustical soundabsorbing material such as mineral wool 37. The absorptive material ispreferably protected behind thin preferable metal sheets or wire screen41. The expansion chamber 27 is located at the end of the motor housingopposite the air intake duct 15. The resonator chamber 31 at leastpartially circumferentially surrounds the expansion chamber with aplurality of openings 43 located between the resonating chamber 31 andthe expansion chamber 27. The amount of noise attenuation achieveddepends on the volume of the resonating and expansion chambers as wellas the size and length of the openings leading in and out of thechambers. The size of the inlet and outlet passageways to the motorhousing is also dictated by the air flow required for cooling purposesthrough the motor.

A schematic diagram showing exhaust air driven by an electric motorin-line fan 11 in a plenum 45 through an expansion chamber 27 is shownin FIG. 4A. FIG. 4B shows an impedance-type analogous circuit of theschematic of FIG. 4A. The impedance-type analogous circuit has a voltagesource P analogous to the acoustic pressure of the fan. The voltagesource is in series with an inductor M₁ which represents the acousticmass displaced by the fan and a capacitor C₁ representing the acousticcompliance of the fan plenum. In parallel with capacitor C₁ are aninductor M_(2i) representing the acoustic mass of the orifice leadinginto the expansion chamber and a capacitor C₂ representing the acousticcompliance of the expansion chamber. In parallel with capacitor C₂ arean inductor M_(2o) representing the acoustic mass of the outlet orificeof the expansion chamber and an impedance Z_(rad) representing theradiation impedance of the outlet of the expansion chamber.

Referring now to FIG. 4C the graph has a horizontal line 47 at 0 dBrepresenting the noise level without the use of an expansion chamber.The curve 49 shows the transmission loss at different frequencies. Thenoise levels are reduced by 10 to 20 dB at most frequencies, but a puretone is produced at the resonant frequency of the expansion chamberwhich is louder than the noise at that frequency without the use of anexpansion chamber. The performance of the expansion chamber isdetermined primarily by its volume and the size of the entrance and exitapertures. The chamber creates an impedance mismatch for the acousticenergy generated by the fan and traveling through the chamber. Theimpedance mismatch results in a reflection of part of the energy backtowards the source of the sound, preventing that part from beingtransmitted outside the expansion chamber exit. The impedance providedby the chamber varies as a function of frequency and at the resonantfrequency the acoustic energy is amplified rather than reduced.

Referring now to FIG. 5A a schematic representation of an electric motorfan 11 exhausting air through a lined expansion chamber 51 is shown. Theimpedance-type analogous circuit is the same as shown in FIG. 4B withthe addition of a resistor R₂ in series with capacitor C₂. R₂ representsthe acoustic resistance of the expansion chamber which is dependent onthe amount, placement, and characteristics of the porous acousticmaterial used as the lining of the chamber.

The graph in FIG. 5C shows the transmission loss for various values ofthe damping factor η which is proportional to the lining resistance R₂.An overall 10-20 dB reduction is obtained at most frequencies with thepure tone at the resonant frequency of the chamber reduced by thepresence of the acoustic lining. The acoustic lining resists themovement of air particles taking energy out of the sound waves and thusproviding wideband noise reduction characteristics.

Referring now to FIG. 6A a schematic representation of an electric motorfan 11 situated in a fan plenum 45 exhausting air through an expansionchamber 27 having a lined resonating chamber 31 out of the flow path isshown. The impedance-type analogous circuit has a voltage source Panalagous to the acoustic pressure of the fan. The circuit elements M₁and C₁, representing the acoustic mass and compliance of the fan plenum,are not shown in this circuit as they have little effect on the silencerperformance. Inductor M_(2i) represents the acoustic mass of theexpansion chamber inlet, resistor R₂ represents the acoustic resistanceof the expansion chamber and capacitor C₂ represents the acousticcompliance of the expansion chamber. In parallel with R₂ and C₂ areseries connected resistor R₃, inductor M₃, and capacitor C₃. Theresistor R₃ represents the acoustic resistance of the resonating chamber31. The inductor M₃ represents the acoustic mass of the orifice leadingto the resonating chamber. The capacitor C₃ represents the acousticcompliance of the resonating chamber. In parallel with R₃, M₃, and C₃are inductor M_(2o) representing the acoustic mass of the expansionchamber outlet and impedance Z_(rad) representing the radiationimpedance of the outlet of the expansion chamber.

The graph 6C shows both the transmission loss due to just the expansionchamber and due to just the lined resonator 57. The lined resonator ismost effective at reducing noise at its resonant frequency whichdetermined by its volume and dimensions of the openings. If the resonantfrequency is selected to be substantially the same as the resonantfrequency of the expansion chamber the pure tone generated by theexpansion chamber can be reduced. The overall noise reduction which isthe sum of the two curves results in a 10 dB noise reduction without anobjectionable pure tone being created. The curves were produced assuminga resonating chamber volume of 61.2 in.³, a damping factor η=0.5 and anaperture leading to the resonating chamber of 14 in.². The expansionchamber was assumed to have a volume of 113 in.³ an, inlet and outletpassageway each having a cross-sectional area of 40 in.²,and the lengthof the passageways from all the chambers of 0.5 inches. The volume ofthe fan plenum was found not to significantly effect the calculations.

While improved performance was obtained with the use of both anexpansion chamber and a resonating chamber in some applications anexpansion chamber above can provide satisfactory noise attenuation.

The foregoing has described an electric motor with an integral silencerwith reduced noise levels at all operating speeds.

While the invention has been particularly shown and described withreference to several embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An electric motor with an integral silencercomprising:a motor housing having a longitudinal axis and defining aninlet aperture at one end and an expansion chamber at the other end,said expansion chamber having an entrance and an exit, the exit of theexpansion chamber providing an outlet aperture for said housing, saidentrance and said exit both having a cross-sectional area taken acrosssaid longitudinal axis which is smaller than a cross-sectional area ofsaid expansion chamber taken across said longitudinal axis; a statorsituated in said housing; a rotor rotatively mounted within said stator;and a fan secured to said rotor and rotatable therewith for pulling airin said inlet aperture through said stator and through said expansionchamber to the housing exterior, said expansion chamber providing animpedance mismatch for the acoustic energy generated by said fanpreventing a portion of the acoustic energy from exiting the expansionchamber.
 2. The electrical motor with integral silencer of claim 1wherein at least one interior surface of said expansion chamber iscovered with a porous acoustical material.
 3. The electric motor ofclaim 1 wherein said motor housing further defines a resonating chamberwhich opens into said expansion chamber with said opening having across-sectional area smaller than the cross-sectional area of theresonating chamber, the air drawn by the fan does not flow through theresonating chamber, the resonant frequency of said resonating chamberselected to substantially match the resonant frequency of said expansionchamber.
 4. The electric motor of claim 3 wherein at least a portion ofthe interior of the resonating chamber is covered with porous acousticalmaterial.
 5. An electric motor with an integral silencer comprising:asubstantially cylindrical motor housing having an inlet aperture at oneend thereof; a substantially cylindrical expansion chamberconcentrically disposed within said motor housing at an end oppositefrom said inlet aperture, said expansion chamber having an entrance andan exit, the exit of the expansion chamber providing an outlet aperturefor said housing; at least one partially annular resonating chamberdisposed within said motor housing and having an opening into saidexpansion chamber; a stator situated in said housing; a rotor rotativelymounted within said stator; and a fan secured to said rotor androtatable therewith for pulling air in said inlet aperture through saidstator and through said expansion chamber to the housing exterior. 6.The electric motor of claim 5 wherein at least a portion of the interiorof said resonating chamber is covered with a sound absorbing material.7. The electric motor of claim 5 wherein said resonating chamber has aresonate frequency substantially matching the resonate frequency of saidexpansion chamber.